1. Field of the Invention
The present invention relates to a surface acoustic wave filter (SAW filter) for use as a high-frequency device in mobile communication apparatuses.
2. Description of Related Art
A surface acoustic wave (SAW) is a wave propagated as a result of concentration of energy near a surface of an elastic body such as a piezoelectric substrate. Electronic devices using the surface acoustic wave, such as filters, resonators, delay lines and oscillators, have been put to practical applications.
FIG. 1 is a schematic view showing an example of the structure of a longitudinally-coupled double-mode resonator filter as one of SAW filters. In the longitudinally-coupled double-mode resonator filter 1, three inter-digital transducers 4, 3, 4 (IDTs) for exciting and receiving a surface acoustic wave are arranged in the direction of propagation of the surface acoustic wave on a piezoelectric substrate 10. Two reflectors 5 for reflecting the surface acoustic wave are provided on both sides of the three inter-digital transducers 4, 3, 4, in the direction of propagation of the surface acoustic wave.
In each of the inter-digital transducers 3 and 4, two IDT electrodes having a number of (omitted in FIG. 1) electrode fingers 12 are disposed to face each other so that their electrode fingers 12 cross at equal intervals. Among the three inter-digital transducers 4, 3, 4, the IDT electrode on one side of the center inter-digital transducer 3 has an input terminal IN of the longitudinally-coupled double-mode resonator filter 1. The IDT electrode on the other side of the center inter-digital transducer 3 is connected to the ground. The IDT electrode on one side of each of the inter-digital transducers 4 on both sides is connected to the ground. The IDT electrodes on the other side of the inter-digital transducers 4 on both sides are connected together and has an output terminal OUT of the longitudinally-coupled double-mode resonator filter 1.
In each reflector 5, a number of (omitted in FIG. 1) electrode fingers similar to the electrode fingers 12 of the inter-digital transducers 3 and 4 are arranged at substantially the same intervals as the intervals in the inter-digital transducers 3 and 4. The three inter-digital transducers 4, 3, 4 and the reflectors 5 have substantially an equal aperture length W that is a length in a direction orthogonal to the direction of propagation of the surface acoustic wave.
In the longitudinally-coupled double-mode resonator filter 1 having such a structure, when an alternating voltage is applied to the input terminal IN, the piezoelectric substrate 10 has a distortion between adjacent electrode fingers 12 as a result of the piezoelectric effect, and the surface acoustic wave is excited. The surface acoustic wave is most strongly excited when its wavelength and the pitch of the electrode fingers 12 are equal to each other. Since the excited surface acoustic wave is reflected by the two reflectors 5, the vibration energy is enclosed between the two reflectors 5, and a standing wave is generated to resonate. This resonated output can be obtained from the output terminal OUT. The standing wave has a dual mode in which a primary-mode wave having a displacement distribution as shown in FIG. 2A and a tertiary-mode wave having a displacement distribution as indicated in FIG. 2B exist.
In the longitudinally-coupled double-mode resonator filter 1 as described above, various filter characteristics are obtained by setting the design parameters. However, regarding the pass band characteristic, in general, the sharpness in the vicinity of a cutoff frequency on the high-frequency side is unsatisfactory as shown in FIG. 3A. Therefore, there is a problem that sufficient attenuation cannot be obtained.
Moreover, a difference in the electrode structure between the input side and the output side causes a problem of poor input and output impedance matching. In FIG. 3B, the reflection characteristics are shown by representing each of the relationship S11 between the incident wave from the input side (IN) and the wave reflected in the filter to return to the input side and the relationship S22 between the incident wave from the output side (OUT) and the wave reflected in the filter to return to the output side by a voltage standing wave ratio (VSWR). The voltage standing wave ratio (VSWR) is the ratio of the maximum voltage and the minimum voltage when the magnitude of the standing wave generated on the propagation path is shown in voltage. As the value becomes closer to 1, the matching state is improved. As shown in FIG. 3B, the characteristic S11 shows a value substantially 2 or less within the pass band and is thus satisfactory, but the characteristic S22 is inferior to the characteristic S11.
In order to solve the above problem, with a prior art, as shown in FIG. 4, the longitudinally-coupled double-mode resonator filters 1 as described above are vertically connected in two stages on a piezoelectric substrate 11. As a result, the vicinity of the cutoff frequency on the high-frequency side was suppressed. However, this structure was suffered from a large insertion loss (difference between the attenuation when the filter was inserted and the attenuation when the filter was not inserted).
As one example of a technique using a longitudinally-coupled double-mode resonator filter with respect to similar problems, Japanese Laid-Open Patent Publication No. 8-191229 (1996) discloses a xe2x80x9clongitudinally-coupled double-mode resonator SAW filterxe2x80x9d. In the xe2x80x9clongitudinally-coupled double-mode resonator SAW filterxe2x80x9d, the inter-center intervals of the facing innermost electrode fingers of a center IDT and of each of side IDTs are different on the right and left sides.
The present invention has been made with the aim of solving the above problems, and an object of the present invention is to provide a surface acoustic wave filter having high sharpness in the vicinity of the cutoff frequency on the high-frequency side of the pass band and capable of obtaining sufficient attenuation and a good input and output matching state.
A surface acoustic wave filter according to the present invention comprises a longitudinally-coupled double-mode resonator filter in which a plurality of inter-digital transducers for exciting and receiving a surface acoustic wave are arranged adjacent to each other in the direction of propagation of the surface acoustic wave, and a resonator having one or a plurality of inter-digital transducers for exciting and receiving a surface acoustic wave. The longitudinally-coupled double-mode resonator filter and the resonator are connected in series. The cutoff frequency on the high-frequency side of the pass band of the longitudinally-coupled double-mode resonator filter and the antiresonant frequency of the resonator are substantially equal.
Two reflectors for reflecting the surface acoustic wave are provided on both sides of the plurality of inter-digital transducers arranged in the longitudinally-coupled double-mode resonator filter, in the propagation direction. Vibration energy of the surface acoustic wave can be enclosed between the two reflectors.
The resonator is a one-port resonator. Two reflectors for reflecting the surface acoustic wave are provided on both sides of the resonator, in the direction of propagation of the surface acoustic wave.
In the surface acoustic wave filter according to the present invention, the longitudinally-coupled double-mode resonator filter and the resonator are mounted on a single piezoelectric substrate. It is therefore possible to realize a surface acoustic wave filter having smaller dimensions.
FIG. 5 is a schematic view showing an example of the structure of a one-port resonator as a resonator using a surface acoustic wave. In this one-port resonator, two reflectors 7 for reflecting a surface acoustic wave are mounted on a piezoelectric substrate 9 on both sides of an inter-digital transducer 6 (IDT) for exciting and receiving the surface acoustic wave, in the direction of propagation of the surface acoustic wave.
In the inter-digital transducer 6, two IDT electrodes having a number of (omitted in FIG. 5) electrode fingers 13 are disposed to face each other so that their electrode fingers 13 cross at equal intervals. The IDT electrode on one side of the inter-digital transducer 6 has an input terminal IN of the one-port resonator 2, while the IDT electrode on the other side has an output terminal OUT.
In each reflector 7, a number of (omitted in FIG. 5) electrode fingers similar to the electrode fingers 13 of the inter-digital transducer 6 are arranged at substantially the same intervals as the intervals in the inter-digital transducer 6.
In the one-port resonator 2 having such a structure, when an alternating voltage is applied across the input terminal IN and output terminal OUT, the piezoelectric substrate 9 has a distortion between adjacent electrode fingers 13 as a result of the piezoelectric effect, and the surface acoustic wave is excited. The surface acoustic wave is most strongly excited when its wavelength and the pitch of the electrode fingers 13 are equal to each other. Since the excited surface acoustic wave is reflected by the two reflectors 7, the vibration energy is enclosed between the two reflectors 7, and a standing wave is generated to resonate.
The one-port resonator 2 has the frequency characteristics as shown in FIG. 6, for example, and has an antiresonant frequency f1. In the present invention, the longitudinally-coupled double-mode resonator filter 1 and the one-port resonator 2 are connected in series, and the cutoff frequency on the high-frequency side of the pass band of the longitudinally-coupled double-mode resonator filter 1 and the antiresonant frequency f1 of the one-port resonator 2 are substantially equal. As a result, the pass band characteristics show higher sharpness in the vicinity of the cutoff frequency on the high-frequency side, and sufficient attenuation is obtained.
In the surface acoustic wave filter according to the present invention, a pitch of electrode fingers in the inter-digital transducers of the resonator is larger than a pitch of electrode fingers in the inter-digital transducers of the longitudinally-coupled double-mode resonator filter. Since antiresonant frequency of the inter-digital transducers of the resonator is adjusted by adjusting the wavelength thereof, the antiresonant frequency of the inter-digital transducers of the resonator is adjusted to a frequency on the high-frequency side of the cutoff frequency of the longitudinally-coupled double-mode resonator filter in the present invention.
Additionally, in the surface acoustic wave filter according to the present invention, the longitudinally-coupled double-mode resonator filter is provided with three inter-digital transducers, since more than two inter-digital transducers are indispensable in order to obtain pass band higher than 75 MHz (with the fractional bandwidth of 4.1 to 4.3%). Since the longitudinally-coupled double-mode resonator filter includes only three inter-digital transducers, which meet the minimal requirements, the number of the design parameters to set for the filter can be reduced.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.